Optical Detection for the Adsorption Kinetics of Surfactant Solutions on the Surface/Interface: Oblique-Incidence Reflectivity Difference Investigation

Abstract

The interaction of interfacial molecules was a hot issue in many fields, such as petroleum: the adsorption kinetics process of surfactant molecules on the interface was the key to reduce the dynamic interfacial tension and enhance oil recovery. Since the aggregation behavior of surfactant molecules on the water surface would lead to changes in the dielectric properties of the surface layer, we applied a dielectric detection technology with a vertical resolution of nanometer precision, oblique-incidence reflectivity difference (OIRD) technology, to study the adsorption kinetics of surfactant molecules on the water surface in this article. According to the test results, we established a model of molecular dynamic adsorption and explained its mechanism. This dynamic process could be divided into three stages: micelles dissociated in bulk water, molecules diffused to the subsurface layer, and molecules broke through the adsorption barrier and adsorbed on the water surface. Among them, the diffusion process was in accordance with Fick’s second law, and the process of surfactant molecules being adsorbed from the subsurface to the surface can be described by the Arrhenius formula. This work was not only applicable to the petroleum field to understand the essence of surfactant reducing surface tension but also to help investigate the subject of molecular behavior at the liquid–liquid or gas–liquid interface in other fields.